1. Field of the Invention
The invention relates to a reflective display device, particularly to such a device based on active matrix control or display of an electro-optic material such as a liquid crystal. The display can be fabricated on integrated circuits based on single crystalline silicon wafers.
2. Description of the Prior Art
Commercial active matrix displays are usually based on light transmission They are matrix devices where the transmission of light of each individual pixel is controlled by an electrical voltage. These active matrix liquid crystal displays are commonly based on the twisted nematic (TN) effect of liquid crystal for controlling light transmission, and semiconductor thin film transistors (TFT) for controlling the individual pixel's transmission state. The TFT can be made of materials such as polycrystalline silicon, amorphous silicon or cadmium selenide.
The principle of these active matrix display devices is as follows: incoming light is first polarized by passing through a polarizer, and then sent into the liquid crystal cell, which comprises many pixels. When no voltage is applied on the pixel, the polarization vector of the incoming light will be rotated by the liquid crystal molecules through the TN effect. A polarizer placed at the output side of the liquid crystal cell can be used to transmit either the light (normally on) or reject the light (normally off). When a proper voltage is applied to the pixel, the TN effect disappears and the polarization of the light is unchanged. The light will therefore be rejected (off) or transmitted (on) by the output polarizer, depending on whether the device is normally on or off respectively. Each pixel can be turned on or off independently by the active control of the voltage across it. Gray scale can also be achieved by supplying intermediate voltages. All of such active matrix displays are invariably based on glass substrates and function in the transmission mode. Polarizers are placed in the front and back of the liquid crystal panel. Some displays are viewed in reflection by placing a mirror in the back of the liquid crystal panel. But the operation is, however, transmission, not reflection.
The active control of the display matrix is achieved by controlling the electrical signals on the transistors on each pixel. Additionally, driver electronic circuits are required to control the timing and scanning of the electrical pulses to each pixel. While TFTS are adequate for controlling the transmission state of the pixels, they are undesirable for the driver part of the active matrix display. The most important reason is that they are based on thin semiconductor films grown on glass substrates, and require special processing techniques to provide good yield in manufacturing. TFT integrated circuits are also not easily designed using standard very large scale integrated (VLSI) circuit design tools in the sense that the current-voltage characteristics of TFTS are different from those transistors fabricated on single crystalline silicon.
There have been many attempts to replace TFT on glass by high quality transistors and circuits fabricated on single crystalline silicon wafers. Lipton et al described an active matrix liquid crystal display based on crystalline silicon in 1978. Dynamic light scattering from the liquid crystal was used as the display mechanism. When there is no voltage on the pixel, the light is transmitted. When a voltage is applied, the liquid crystal becomes turbid and scatters the incoming light. Unfortunately, this device has poor contrast and light transmission efficiency. Yamasaki et al described a singular crystalline silicon based device where the display is based on the guest-host effect in dye doped liquid crystals. In this display, the absorption coefficient of the dye is dependent on the orientation of the dye molecules. These guest molecules tend to align themselves with the host liquid crystal molecules. Hence the absorption of the guest/host combination can be controlled by an applied voltage.
The concept of dynamic scattering for display was revived recently due to the invention of polymer dispersed liquid crystals. The liquid crystal droplets dispersed in a polymer film can have very high scattering efficiency, and a high contrast ratio between the on (no scattering) and off (scattering) states. It is therefore a potentially important technology for providing active matrix displays on crystalline silicon. In 1990 Drzaic et al described a reflective active matrix liquid crystal display based on such polymer dispersed liquid crystals. However, there are drawbacks for this technology such as the stability of the liquid crystal polymer and the need to use higher voltages to operate such a display. The reason for the latter requirement is simply that the polymer films cannot be made to be the same thickness (a few microns) as the liquid crystal film in an active matrix liquid crystal display. It is the electric field (voltage divided by thickness) that is important in aligning the liquid crystals.